Move the test of minimum supported keepalive slots to CTS

This change also enables log when keepalive is started.

Bug: 134352656
Test: 1. atest android.net.cts.ConnectivityManagerTest#testSocketKeepaliveLimitTelephony
      2. atest FrameworksNetTests
Change-Id: I408750fa0bceb0c1c26afb5fead4e44fb824fbc1